The field of the invention pertains to thermoelectric generators and methods of operating such generators.
Thermoelectric materials, and generators grounded in the use of such materials, have engendered a great deal of interest over many years. Such materials generate electrical power in response to temperature differences therealong. In particular, a piece of thermoelectric material will generate a voltage (the Seebeck voltage) between two points therealong dependent upon the temperature difference between such points.
In light of this, the widespread interest in thermoelectric generators is readily understandable. Heat, to create the required temperature differences, is readily available in a variety of forms. Electrical power, which can be generated by thermoelectric generators from such heat, is highly valued.
Typically, to generate a thermoelectric voltage across a piece of thermoelectric material, a steady state, generally linear, temperature profile is established along the piece of material to provide a steady state voltage.
Due to efficiencies which have been considered unacceptably low for all but specialized applications, substantial efforts have been made to improve the performance of thermoelectric generators. Most of these efforts can be classified into four categories: (1) efforts to permit higher temperatures (and thus temperature differences) in the thermoelectric material, largely through a search for better materials; (2) efforts toward the utilization of readily available heat sources, toward the improvement of the characteristics and efficiencies of heat sources and toward convenient ways of taking and using the electrical power from the generators; (3) efforts directed to the detailed structures of, or intimately associated with, the thermoelectric material elements of the generators; and (4) efforts toward the efficient use, recovery and reuse of heat which is required by the generators.
Concerning the first of these categories, the classical goal is to find or develop materials having a high Figure of Merit. This Figure of Merit is, in general, taken to be a measure of the capability of a thermoelectric material to efficiently convert temperature differences to electrical power. The Figure of Merit (Z) is a function of the Seebeck Coefficient (S), the electrical conductivity (.sigma.) and the thermal conductivity (k) of the material, as follows: EQU Z=S.sup.2 (.sigma./k),
where
S is in units of volts/(degree C), PA1 .sigma.is in units of mho/(cm), and PA1 k is in units of cal/(sec-cm-degree C).
This search is generally accompanied by a search for materials which, at the same time, can tolerate higher temperatures (and thus higher temperature differences) to, in effect, be converted to electrical power.
Chad, U.S. Pat. No. 3,783,031, Hill et al., U.S. Pat. No. 3,138,486, Cody et al., U.S. Pat. No. 3,279,954 and Hampl, Jr. et al., U.S. Pat. No. 3,873,370, are typical and illustrative of the concern and efforts directed to this first category. In particular, Chad indicates the existence of the problem of evaporation of dopants and of base materials in thermoelectric cells operating at elevated temperatures. Hill indicates the desire for higher temperature thermoelectric materials--in the case of Hill, in the range of from about 800 degrees centigrade to 1500 degrees centigrade. Cody similarly indicates the desire for higher temperature materials--in Cody, in the range of about 1200 degrees kelvin. Hampl is along similar lines in showing the desirability of higher temperature operation capabilities--in Hampl, in the range of about 1000 degrees centigrade.
The second of the above categories pertains to the form or characteristics of the heat source which provides heat to the thermoelectric generator or of the removal and utilization of the electrical power provided by the generator. With respect to this category, Findley, U.S. Pat. No. 2,362,259, Underwood, U.S. Pat. No. 2,015,610, Creveling, U.S. Pat. No. 1,118,269, Hale, U.S. Pat. No. 1,134,452, Pepper, U.S. Pat. No. 3,297,492, Hanson, U.S. Pat. No. 4,095,998, McCollum, U.S. Pat. No. 2,391,994, and Harkness, U.S. Pat. No. 3,090,875, are typical and illustrative.
Findley incorporates a pulsed pump to provide pulses of fuel when the back pressure in the fuel line is low. This is viewed as a convenient way of providing fuel to a combustion chamber. In addition, current from an array of thermoelectric cells is employed to open and close a magnetic (solenoid-type) relay.
Underwood monitors the voltage of a thermoelectric generator to open and close (through use of a solenoid) the supply of fuel, in order to maintain the voltage in a desired range. This approach is adapted to maintain the voltage substantially constant and, also, to be adjusted so that the fuel supply is not rapidly cut-off, opened, cut-off, etc.
In Creveling, a thermoelectric generator is driven by an internal combustion engine (i.e., a combustion engine employing reciprocating cylinders in the generation of power). Current from the generator is used in a solenoid-type relay (and to charge a battery and power other devices). In McCollum, the internal resistance of a thermoelectric generator is used to heat and, thereby, de-ice airplane wings. High resistance metals in the wings, as a load on the generator, are similarly employed.
In Harkness, current from a thermoelectric generator (to which heat is provided from a burner) is passed through the rotor of a motor to provide the current through the rotor which results in the rotation of the rotor. The rotor is a disc or a disc with internal spokes. The motor in Harkness uses permanent magnets, or electromagnets which, it is noted, could be driven by a thermoelectric generator. Hale and Pepper are of interest in providing an internal combustion engine as a source of heat for a thermoelectric generator. Hanson, similarly, employs a vehicle engine.
Still with regard to this second category, Kolm et al., U.S. Pat. No. 3,198,969, Bauer, U.S. Pat. No. 3,421,944 and Peck, U.S. Pat. No. 4,211,828 are of some, peripheral interest.
Kolm is directed to the utilization of pyroelectric, including piezoelectric, effects in materials such as ceramics to develop pyroelectric energy. Expansion of the material, with heating, and contraction of the material, with cooling, such expansion or contraction being toward or away from a heat sink which contributes to such cooling, is employed. Such heating and cooling also provides the pyroelectric energy. In one form, cylindrically arranged pieces of pyroelectric material rotate toward and away from a heat source, such as the sun. The energy is removed from the material by a commutator arrangement. In another form, a cylindrical configuration of pyroelectric material is disposed about a rod which is heated by a hot gas source. As a result, the material expands away from the shaft toward a heat sink and, due to the approaching of the heat sink, subsequently contracts back toward the shaft. Switches and associated circuitry can be used to remove the pyroelectric energy from the material.
Bauer is directed to a pulse battery having structure designed with a view to limiting negative factors, such as resistive loss.
Peck is directed to the reduction or elimination of the conventionally decreasing output over time in conventional thermoelectric cells using an electrolyte for conduction between the electrodes.
The third area in which much of the effort toward improving the performance of thermoelectric generators has been made, concerns the detailed structures of, or associated with, the thermoelectric material elements in such generators. Stachurski, U.S. Pat. No. 3,356,539, Stachurski, U.S. Pat. No. 3,899,359, Underwood, U.S. Pat. No. 2,015,610, Falkenberg et al., U.S. Pat. No. 3,719,532, Hale, U.S. Pat. No. 1,134,452, Pepper, U.S. Pat. No. 3,297,492, Winckler et al., U.S. Pat. No. 3,048,643, Krake et al., U.S. Pat. No. 3,358,162, and Hampl, Jr., U.S. Pat. No. 3,873,370, are typical and illustrative of efforts in this area.
In Stachurski, U.S. Pat. No. 3,356,539, a sandwich of thin thermoelectric material structure and thin conducting plates is used in segments along a leg of one type of material, in providing a desired temperature gradient along the leg.
Stachurski, U.S. Pat. No. 3,899,359 incorporates thermoelectric material in sandwiches between contacts, such sandwiches being used in forming segments of a leg of material of one type, in order to provide a temperature gradient along the leg. Somewhat unconventional thin structures are also employed.
Underwood is directed to deriving the advantages of relatively short thermoelectric material elements. Heat conducting structure is also provided which is generally disposed parallel to face-to-face ends of thermoelectric material elements of different materials. Such thermoelectric material elements are bent into three segments, including such end segments.
Falkenberg provides generally conventional heat conduction vanes with generally conventional series connections of thermoelectric material elements.
Hale provides a sandwich of alternating thermoelectric materials, largely insulated by insulation materials therebetween. The ends of the thermoelectric material elements have holes for the passage of heat conduction fluid therethrough.
Pepper is directed to a substantially conventional thermoelectric material arrangement. The cold contacts of the arrangement have relatively large fins therealong.
Winckler, apparently, provides a piece of P-type material, specially configured N-type material, and a contact which also may be specially configured, in, essentially, a three-piece sandwich in which the P- and N-type materials are next to one another. The special configuration for the N-type material is to provide reduced thermal conductance.
In Krake, two different porous, electron-emitting materials are provided in a four-piece sandwich between electrodes which are at different temperatures. A thin metal layer can be provided between the materials, to prevent diffusion between the materials. In a different embodiment, a single sensitive material version, it is indicated that at low temperatures, classical thermoelectric power generation is provided, but at higher temperatures, conduction takes place through the electron gas resulting from the emission of electrons.
Hampl is directed to a thermoelectric material element incorporating migrating atoms, which is divided into segments with barriers between the segments. The separate segments, as indicated, are part of a one-material leg.
Kolm, U.S. Pat. No. 3,198,969, as described above, Marinescu, U.S. Pat. No. 4,039,352, Heard, Jr., U.S. Pat. No. 3,547,705, and Reich et al., U.S. Pat. No. 3,564,860, are of some, peripheral interest with regard to this area.
Marinescu provides a closely-bonded and closely-packed series arrangement to generate electrical power, wherein such power generation is predicated on the difference in work function between metals disposed on either side of a semiconductor. Operation is grounded in the uniform heating and temperature of the power-generating elements.
Heard addresses the undesirability of joints from the perspective of wasted energy in a thermoelectric unit. Structure therein is directed to reducing or avoiding such waste by using crystal growth to provide such joints. The structure is particularly adapted for Peltier--i.e., thermoelectric heating and cooling--units.
Reich is also particularly adapted for Peltier unit applications in providing thermoelectric material legs having changing thermoelectric properties therealong. Peltier units incorporating such legs are arranged as cascade stages of a multi-unit system.
The fourth category in which many efforts to improve the performance of thermoelectric generators falls, relates to the efficient use, recovery and reuse of the heat required to drive thermoelectric generators, particularly with reference to heat-carrying fluids, such as combustion gases. Typical and illustrative references of interest to those who focus on this area are: Stachurski, U.S. Pat. No. 4,125,122, Stachurski, U.S. Pat. No. 3,356,539, Stachurski, U.S. Pat. No. 3,899,359, Guazzoni et al., U.S. Pat. No. 4,218,266, Underwood, U.S. Pat. No. 2,015,610, Falkenberg et al., U.S. Pat. No. 3,719,532, Garnier, U.S. Pat. No. 3,349,248 (thermionic power generation), and Payne, U.S. Pat. No. 3,666,566. A number of these, of course, have been noted above in other contexts to which they also pertain.
Other references, as follows, are considered to be of some, limited relevance to the subject matter herein: Acheson, U.S. Pat. Nos. 375,408 and 407,762, Krebs, U.S. Pat. No. 3,859,143, Kim, U.S. Pat. No. 3,794,527, Rittmayer et al., U.S. Pat. No. 3,900,603, and Bressler, U.S. Pat. No. 3,508,974.
Despite the evident effort directed to improving thermoelectric generators, in the past, as indicated, such generators have not been considered satisfactory for general applications, but only for specific and narrow uses.
The present subject matter is directed to the second and third of the above categories of effort relating to thermoelectric generators. Thus, it is, first of all, directed to the manner in which thermoelectric generators are operated and the manner in which the electrical power they generate is taken from the generator and utilized--including the interaction between the generator and its heat source and the interaction between the generator and the apparatus directed to the recovery and use of the electrical power generated. It, secondly, is directed to the detailed structures of, and intimately associated with, the thermoelectric material elements in thermoelectric generators. It also, within the context of the second and third categories, pertains to the first and third categories, in particular to thermoelectric material choices and to efficient use, recovery and reuse of the heat required by thermoelectric generators. Moreover, it is directed to these matters in an integrated, comprehensive fashion.